The present invention relates to a sound resonator for amplifying sound waves emanating from a sound source, especially a stringed instrument, in which the resonator has the shape of a box, the side walls of which are formed by a supporting frame and the bottom of which is a sounding board and into which is mounted an electromagnet composed of a coil and an armature. The armature is coupled to the sounding board and the coil is intended to be connected to an amplifier or a pressure-sensitive microphone, preferably of crystal type. A device of this type is known from e.g. applicant's Swedish Pat. No. 364,589, U.K. Pat. No. 1,408,895, and U.S. Pat. No. 3,908,503, according to which the sound resonator, also called soundboard, is intended to be used in connection with stringed instruments such as guitars, violins, pianos, etc., which are provided with soundboards, the soundboard of the sound resonator being tuned to that of the instrument in such a manner that the tunes emitted from the instrument are reproduced in amplified form while the beauty of the tune is maintained and even improved while simultaneously overtones are separated off.